In the field of the microfabrication represented by the production of semiconductor integrated devices such as VLSI, due to a trend of ultra high integration of an integrated circuit, ultrafine pattern formation in a range of submicron to quarter micron is demanded. Along with this, with regard also to a exposure wavelength, in order to obtain a higher degree of integration by the g-line to the i-line, a light source with wavelength shorter than an ArF excimer laser (wavelength 193 nm), a F2 excimer laser (wavelength 157 nm), or the like is now used. In addition, by combining with a technology such as immersion lithography, a lithography technology capable of the microfabricating at the level of about 50 nm or less is required. In a base polymer used for a radiosensitive resin composition suitable for photolithography by such excimer lasers, more advanced qualities such as control of molecular weight and molecular weight distribution (for example, see Patent Document 1); stability; homogeneity of distribution of copolymer composition; and reduction of ultrahigh molecular weight components, low molecular weight components and an amount of other impurities, are demanded due to development of miniaturization.
Meanwhile, in the production of a polymer, a composition of a polymer to be generated may be biased due to a difference in the reaction mode. The polymer with the biased composition is produced because the polymerization reaction rate of each of several types of raw material monomers varies. In the reaction mode of the batch style (lumped style) in which the monomers are allowed to react after supplying all raw material monomers to a reactor, a copolymer containing a higher amount of high reactive monomers than an average composition is generated in the initial stage. A copolymer with a composition close to the average composition is generated in the middle stage. A copolymer containing a higher amount of low reactive monomers than the average composition is generated in the final stage.
In order to improve such a situation, thus far various studies have been made. For instance, a dropping polymerization method in which the raw material monomers are continuously supplied is known. The dropping polymerization method is a reaction mode in which a solvent alone, or solvent and only a part of raw material monomers is (are) fed in the reactor and, after it is heated to a reaction temperature, the remaining large part of the raw material monomers is slowly supplied to the reactor over hours. Also, in the dropping polymerization method, although the copolymer produced in the initial, middle, and final stages has the biased composition, since a time period corresponding to the middle stage is relatively longer, compared with that in the batch style, the percentage of a copolymer with the composition close to the average composition, which copolymer is generated in the time period of the middle stage, is high in a mixture of the copolymers. As a result, the influence of copolymers with the biased composition produced in the initial and final stages is small. And thus, improvement of solubility of the copolymer in a resist solvent and an improved effect of a defect and LER are observed.
Furthermore, in order to improve the above-mentioned situation, in the method for production of the copolymer for resists containing at least two or more kinds of monomers, improvement of the solubility of the copolymer in the resist solvent or the like by setting the percentage of a triad of each monomer not more than a specific amount has been proposed (for example, see Patent Literature 2).
However, due to the miniaturization of a photoresist pattern in recent years, necessity for further improvement of defects and LER is imminent. Therefore, a copolymer with a less biased composition than ever is demanded.
In particular, in the photoresists, the biased monomer composition ratio of a base polymer affects improvement of the solubility in the resist solvent, defects, LER or the like. Thus, in the production process of the base polymer, the homogeneity (to be less biased) of the monomer composition ratio is demanded. Hence, it has been desired to provide a base polymer with a less biased monomer composition ratio.    Patent Literature 1: JP 11-295894 A    Patent Literature 2: JP 2003-246825 A